Journal of Applied Microscopy. 281 



stages. Ova at these ages were in almost all cases already in the folds of the 

 uterine wall, and the method of procedure in obtaining and preserving material 

 was as follows : 



Shortly after the death of the animal, three-fourths of an hour to three hours, 

 the upper end of the uterus was filled as completely as possible with .25 or .5 per 

 cent, chromic acid solution, the injection being carried as far as possible to stretch 

 the folds of the lining mucous membrane of the organ, and so float the ova free 

 into the liquid. After ligaturing the lower end, the whole preparation was put in 

 .5 per cent, chromic acid for two or more days. Later, the contents of the uterus 

 were allowed to flow out into a glass and the ova found by careful search with 

 the microscope. Chromic acid was found most satisfactory, owing to its clear- 

 ness and the absence of any precipitate. Flemming's solution was used in some 

 cases with equal success, but Perenyi or nitrate of silver caused too much precip- 

 itate and prevented the finding of ova. x\ll the specimens were stained in toto 

 in either haemalum, carmalum, heematoxylin, or borax carmine, and imbedded in 

 paraffin by the cedar oil me"thod. 



About one hundred embryos were obtained, thirteen of which were less than 

 five-celled and twelve more preblastodermic. The most noteworthy point in con- 

 nection with cleavage is that after the two-celled stage the blastomeres become 

 markedly unequal in size, especially in from eight to twelve-celled stages. No 

 difference is marked beyond that of size and the smaller segments are arranged 

 as a cap on the larger ; the inequality is more marked than in other mammals so 

 far studied. 



The cavity of the blastodermic vesicle is formed by vacuolation of the inner 

 larger cells of the morula, not by the formation of an intercellular cavity. This 

 makes the development of the pig more closely resemble that of the rat, accord- 

 ing to Robinson, in this particular point. As evidence of this process, is given 

 the intense vacuolization of the cells at this stage, the existence of strands of 

 protoplasm reaching across the cavity after its formation and the position of oil 

 globules such as are in the cells, free in the cavity. The end of this stage is 

 marked by the rupture of the zona radiata. This latter event is attended by a 

 great change in the form of the embryo, owing to relief from pressures. The 

 hitherto spherical embryo becomes collapsed and irregular in outline and remains 

 so till a considerably later stage. It is now a vesicle whose walls are one cell 

 thick, forming a layer known as the trophoblast. 



At one point on the inner wall of the trophoblast is a small mass of cells that 

 is the future epiblast of the embryo. 



During the eighth and ninth days changes are not remarkable, but already 

 the hypoblast is recognized as separating from the epiblest, both layers being 

 enclosed in a vesicle of trophoblast. On the tenth and eleventh days growth of 

 the inner layers results in a rupture of the trophoblast, and the epiblast comes 

 to the surface. In later stages masses of cells are found adhering to the out- 

 side of the blastodermic vesicle, which are considered the remains of the tropho- 

 blast. These cells do not necessarily die on rupture of the vesicle, but may 

 persist for some time and even multiply. There is no evidence that they ever 

 fuse with the epiblast. . 



